In general, an image sensor is a semiconductor device for converting optical images into electric signals, and is classified into a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
According to the related art, a photodiode (PD) is formed on a substrate by implanting ions into the substrate. However, as the size of the PD is gradually decreased such that the number of pixels is increased without the increase of a chip size, the size of a light receiving part is reduced so that an image quality may be degraded.
In addition, there has been suggested a method for increasing the electron generation rate by increasing capacitance of the PD. However, expanding a depletion region of the PD to increase the capacitance is limited, and the light aperture ratio may be lowered due to the structure formed in the back end process of the PD.
As an alternative method to overcome the above problems, there has been suggested a method of forming the PD by depositing amorphous silicon (Si). In addition, there has been suggested a structure, in which a readout circuitry is formed on a Si-substrate (main substrate) through a wafer-to-wafer bonding scheme, and the PD is formed on another substrate (donor substrate) provided on the readout circuitry (hereinafter, referred to as “PD-up CIS” or “three dimensional (3D) image sensor”).
Such a structure can be obtained by bonding the donor substrate with the main substrate after sequentially forming a p+ area, an n− area, and an n+ area on a PD region of the donor substrate.
According to the above structure, the light aperture ratio can be improved and the depletion region (p-area) of the PD can be expanded. Thus, higher capacitance can be achieved, resulting in a higher electron generation rate being obtained.
However, defects may occur in the process of bonding the donor substrate with the main substrate. For instance, a contact defect may occur in a contact plug, which connects the PD of the donor substrate with metal, or the insulating property may be degraded between components of the Si-substrate.
In this case, current generated from the PD of the donor substrate may not be easily transferred to the readout circuitry of the main substrate due to high resistance of about 105Ω to 108Ω, so the operational reliability of the image sensor may be degraded.